Structure Maps and Phase Stability in AlTi 3 Alloyed with Rare-Earth Elements

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STRUCTURE MAPS AND PHASE STABILITY IN A1Ti3 ALLOYED WITH RARE-EARTH ELEMENTS* C. T. LIU AND J. A. HORTON Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115 D. G. PETITIFOR Department of Mathematics, Imperial College of Science and Technology, London, England ABSTRACT Rare-earth elements including Y, Er and Sc were added to AlTi3 for stabilizing the L12 ordered crystal structure, as predicted by the AB3 structure map. The crystal structure and phase composition in the AITi3 alloys were studied by electron microprobe analysis, X-ray diffraction and TEM. The solubility limit of the rare-earth elements were determined and correlated with the atomic size factor. The results obtained so far indicate that rare-earth additions are unable to change the crystal structure of A1Ti3 from DO 19 to L12. The inability to stabilize the L12 structure demonstrates the need to characterize the structure map domains with a further period-dependent parameter. INTRODUCTION The stability of ordered intermetallic phases has been correlated with four physical factors: electronegativity difference (AX), atomic size difference (AR), average number of valence electrons per atom (e/a), and angular dependence of the valence orbitals [1-6]. The last factor is related to the quantum character of electrons, which is generally neglected by classic approaches [4]. Recently, Pettifor has constructed structure maps for binary intermetallic compounds, using a single phenomenological parameter, namely the Mendeleev number (M), assigned to each alloying element based on its position in a modified periodic table [6-8]. This number, in principle, includes all alloy information from both classical and quantummechanical considerations. The merit of Pettifor's scheme is that structure maps for intermetallic compounds can be represented by two-dimensional plots. An example of the structure maps is shown in Fig. 1 for AB 3 compounds [4,7,8]. The structure map successfully groups different structure types into separate domains in the map. Furthermore, ordered crystal structures observed in pseudobinary and ternary intermetallic alloys fit well into the domains of structural stability for binary intermetallic phases [4]. This indicates that the binary structure maps are able to serve as a useful guide in control of ordered crystal structures in multi-component intermetallics. The titanium aluminide AlTi 3 is of interest because of its low material density, good high temperature strength, and adequate oxidation resistance at elevated temperatures. The aluminide is, however, brittle at ambient temperatures [9,10]. The brittleness is related to its hexagonal ordered crystal structure (DO 19), which has limited deformation modes. On the other hand, A1Zr3 with the L12 structure is quite ductile at room temperature [11,12]. A possible way to improve the ductility of AITi3 is thus to change its ordered crystal structure from hexagonal DO 19 to cubic L12. In this study, the AB 3 structure map is used as a guide for control of the ordered

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